1. Field of the Invention
The present invention relates to a controlling device of a tunable filter which selectively transmits rays of a predetermined wavelength of an incident light, and more particularly, to a controlling device for controlling the angle that the incident rays make with the line normal to the surface of the tunable filter.
2. Description of the Related Art
Generally, a tunable filter including a line filter is used to select a ray of a specific wavelength in an optical communication system employing a wavelength division multiplexer (WDM), an erbium doped fiber amplifier (EDFA) or the like.
Referring to FIG. 1, a conventional controlling device of a tunable filter comprises a holder 8 in which a tunable filter 5 is installed, a DC servo-motor 11 for driving the tunable filter 5, an encoder 13 fixed to a rotating shaft 11a of the DC servo-motor 11, and reduction gears 15 for changing the rotational speed of the servo-motor 11 to a suitable speed. The encoder 13 measures the angle of disposition of the tunable filter 5 with respect to the incident rays by detecting the rotation angle of the DC servo-motor 11.
The DC servo-motor 11 controls the angle of disposition of the tunable filter 5 with respect to the incident rays. The tunable filter 5 selectively transmits a ray of a specific wavelength among the rays emitted from a first optical fiber 1 and directs the selected rays to a second optical fiber 9.
The reduction gears 15 installed between the DC servo-motor 11 and the tunable filter 5 include a center gear 17 fixed to the rotating shaft 11a of the DC servo-motor 11, first, second and third gears 18a, 18b and 18c meshed with the center gear 17, a ring gear 19 having teeth 19a on the inner circumferential surface thereof to mesh with the first, second and third gears 18a, 18b and 18c. A rotating shaft 19c provided at the center of a cover 19b of the ring gear 19 is fitted to the holder 8. In order to drive the tunable filter 5 in fine movements, a plurality of the reduction gears 15 as described above are provided.
As the center gear 17 is rotated according to the rotation of the DC servo-motor 11, the first, second and third gears 18a, 18b and 18c meshed with the center gear 17 are simultaneously rotated. Since the first, second and third gears 18a, 18b and 18c are meshed with the ring gear 19, they revolve around the center gear 17. The ring gear 19 is rotated at a speed that is equivalent to the revolution speed of the first, second and third gears 18a, 18b and 18c.
Accordingly, the ring gear 19 is rotated at a speed slower than that of the DC servo-motor 11, and at the same time the holder 8 fitted to the rotating shaft 19c of the ring gear 19 is rotated at the same slower speed, and therefore the angle of disposition of the tunable filter 5 can be finely controlled. Consequently, since the angle at which the tunable filter 5 is disposed can be appropriately controlled according to the value measured in the encoder 13, a ray of specific wavelength can be selectively transmitted.
However, since the controlling device of the tunable filter is provided with a plurality of reduction gears, the structure of the control device is complicated and the size of the whole controlling device is large.